Devices for the high-temperature granulation of thermoplastic materials comprise, in general:
(a) a die, constituted by a heated perforated body;
(b) a chopping device comprising a rotary drive shaft, provided with a blade-carrier disc, and with a system for positioning the blades against the outer face of the die;
(c) a granule-collecting chamber; and
(d) a system for feeding and distributing a temperature-controlled fluid, generally water, necessary for cooling and conveying out of the collecting chamber the chopped granules.
The thermoplastic material in the molten state is extruded through the bores of the die; the monofilaments exiting the die are cut by the rotary blades and the so-obtained granules are cooled and removed by means of the temperature-controlled fluid.
The cutting device generally comprises a cutting plate, constituted by the outer face of the die; and blades, mounted on a blade-carrier disc keyed on the drive shaft, driven to revolve in front of the cutting plate, at a distance adjusted by means of an axial-shift device.
Several high-temperature cutting devices are known which however do not completely fulfill all of the technological requirements.
Thus, for example, cutting devices are known which are equipped with rigid blades integral with the drive shaft. In these devices, the adjustment of the position of the blades is difficult and not precise, because of the axial stiffness of the whole, and, in practice, in as much as it is not possible to achieve a sufficient uniformity in the behavior of the blades, a poor quality of cutting and/or severe wear of the cutting elements results.
In order to overcome these drawbacks, cutting devices have been proposed which are equipped with elastic blades, or wherein the blade-carrier disc is axially loaded by means of springs.
Because of the sliding contact between the blades and the die face, in these devices a wear of a diffused type occurs.
Furthermore, these cutting devices of the prior art are not suitable to be indifferently applied for all types of thermoplastic polymers. In fact, the devices equipped with rigid blades are not suitable for use for relatively soft or low-viscosity polymers, because, in as much as the blades must be adjusted in a position very close to the face of the die, in practice areas of irregular blade-plate contact occur, with resulting very intense wear.
Devices equipped with elastic blades, or with a spring-urged blade-carrier disc, on the contrary, are not suitable for polymers with relatively high values of viscosity and hardness, in that, in as much as a high thrust of the blades against the plate is necessary, they show a quick wear.
In the Italian patent applications No. 21,325 A/84 and No. 22,649 A/84, cutting devices have been proposed which comprise a blade-carrier disc rigidly keyed on the drive shaft, and sliding blades applied to the disc in a non-rigid fashion, but associated with inertial elements, e.g., relatively heavy masses movable inside guides provided in the same blade-carrier disc. These devices, even if they make it possible to satisfactorily solve the problem of the wear of the cutting elements, have been shown to be not very suitable for the granulation of thermoplastic materials, because at the high speeds the moving parts do not succeed in adapting themselves to the die.